Nitrogen dioxide gas-sensing properties of hydrothermally synthesized WO3 · nH2O nanostructures

Nitrogen dioxide (NO2) has been identified as a serious air pollutant that threats to our environment, human life and world ecosystems. Therefore, detection of this air pollutant is crucial. Metal oxide semiconductor is one of the best approaches frequently used to detect NO2 at relatively low temperatures. Hydrated tungsten trioxide (WO3 · H2O), an n-type semiconductor, is regarded to be a promising material for fabricating gas sensors, which are widely used in environmental and safety monitoring. In this work, WO3 · nH2O nanoparticles have been synthesized using a polyfunctional surfactant-mediated hydrothermal approach in the addition of H2C2O4 and K2SO4 at a molar ratio of 1 : 1. This paper has also reported the effect of reaction temperature (120°C to 200°C) on morphological changes and gas-sensing performance. The characterization of these synthesized nanostructures was carried out by UV–Vis absorption spectroscopy, X-ray diffraction and field-emission scanning electron microscopy (FESEM). The UV absorption peak was obtained around 300 nm. FESEM analysis showed sheet-like structures come together to form flower-type morphology. The synthesized WO3 · nH2O flower-like structures was then used for NO2 gas-sensing application. The prepared sensors showed considerably better sensor response (Rg/Ra = 17.48) at 185°C for 25 ppm NO2.


Introduction
Nitrogen dioxide (NO 2 ) gas is one of the main air pollutants that contributes to respiratory infections, acid rain and depletion of ozone layer [1,2]. NO 2 is emitted by the combustion of fossil fuels and vehicle exhaust that not only causes serious respiratory issues in humans but it can also produce a variety of noxious gases [3,4]. According to an alert from the National Institute for Occupational Safety and Health (NIOSH), NO 2 can cause death when present at concentrations more than 20 parts per million. Thus, it is necessary to fabricate a high-performance gas sensor which can rapidly, precisely and reliably detect low concentration of NO 2 in the air. Several types of gas sensors (electrochemical, optical and resistive) have been extensively applied for real-time NO 2 sensing [5][6][7][8][9][10]. However, gas sensors based on metal oxide semiconductors (MOSs) have low production costs as well as high sensitivity and selectivity towards desired gases and hence gained a lot of interest in environmental pollution monitoring [11]. Certain semiconductor oxides, including SnO 2 , ZrO 2 , ZnO, CeO 2 , In 2 O 3 , TiO 2 and WO 3 , are commonly used for their high sensitivity towards specific gases and their ease of manufacturing [12][13][14][15]. Especially, tungsten oxide (WO 3 ) and its hydrates (WO 3 · nH 2 O) are one of the most effective n-type MOSs that has been discovered as a potential material for NO 2 gas monitoring [16]. In the case of gas sensors, morphology, particle size and solubility rate of nanomaterials are all important factors. Therefore, the well-controlled morphology of WO 3 · nH 2 O needs to be evaluated first, and based on this, research was focused on study of several forms of WO 3 · xH 2 O such as nanowires, hollow sphere, square-like micro-and nanostructures, nanoparticles, and nanorods [17][18][19][20]. This morphology could effectively increase the surface for gas adsorption and desorption, leading to improvement in the gas-sensing performance. To date, several methods such as chemical vapour deposition [21], spray pyrolysis [22], sol-gel [23] sputtering [24], hydrothermal [25], solvothermal approach [26] and other techniques were used to synthesize WO 3 nanoparticles. The hydrothermal technique is one of the important and commonly used methods for manufacturing of nanoscale materials due to its low-temperature range, easy process management etc. Additionally, it is possible to achieve the improved crystallinity without heat treatment [27,28]. A hydrothermal method is also used to develop free-standing nanostructures with a number of morphologies at low temperatures. These nanostructured materials are promising candidates for gas-sensing applications [29,30].
According to Chung et al. [31] and Lee et al. [32], WO 3 -based gas sensors with a large surface area have shown a great sensitivity towards NO 2 gas. Similar to this, Stankova et al. [33] have reported WO 3 thin film deposition by RF sputtering, and the gas response noted was about 5 at 1 ppm of NO 2 gas at 370°C of operating temperature. While An et al. [34] found that hydrothermally synthesized one-dimensional WO 3 nanorods exhibited good gas-sensing response at 300°C.
An efficient ethanol gas sensor was developed by Liu et al. [35] by using WO 3 · H 2 O nanorods and spherical networks that were synthesized by a simple hydrothermal process. It has been shown that the spherical network has superior gas sensitivity in comparison with the evenly spread nanorods. This may be due to the porous structure being more effective than the uniformly distributed WO 3 nanorods, and the greatest sensing response was observed for 100 ppm ethanol at 350°C. Zeng et al. [36] synthesized WO 3 · H 2 O with diverse morphologies using a hydrothermal process with various number of surfactants. The optimized composition showed great sensing response to ethanol at 350°C for 400 ppm. WO 3 and WO 3 · nH 2 O-based gas sensors worked well when operated at high temperatures, as reported earlier. Due to this, an attempt was made throughout this study to develop a WO 3 · H 2 O-based gas sensor that could function at temperatures lower than their usual operating temperature. The effectiveness of gas sensing is also discussed in relation to the morphological changes that happened in WO 3

Synthesis and fabrication of flower-like tungsten oxide nanostructures
In this synthesis method, 5 mmol Na 2 WO 4 · 2H 2 O was mixed in 120 ml DI water with constant stirring so as to make a transparent solution, and then a 3 M HCl solution was added drop-wise into the transparent solution until the pH value reached 1. Following this step, a combination of 5 mM K 2 SO 4 and 5 mM H 2 C 2 O 4 was added to the previously mentioned mixture. Following that, the mixture was transferred royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 221135 to 200 ml Teflon-lined stainless-steel autoclave and held at 120°C, 160°C and 200°C for 12 h. After gradually cooling to room temperature, the supernatant was taken and rinsed several times with DI water and ethanol before drying for 12 h in an electric oven at 60°C. The synthesized samples are labelled as W1, W2 and W3.

Characterization
X-ray powder diffraction patterns produced using Bruker D8 Advance X-ray diffractometer with bandpass filter Cu Kα radiation (λ = 1.54 Å) in the 2θ = 20°−80°range are used to evaluate the crystalline phase of the synthesized samples. Material's optical spectra were measured with a UV-Vis absorption spectroscopy spectrometer (Jasco V750). To analyse the morphology of the synthesized WO 3 nanomaterial, field-emission scanning electron microscopy (FESEM) was carried out using a FET Nova Nano SEM 450. Fourier transform infrared (FTIR) spectroscopy measurements (Shimadzu FTIR-8900) were taken in the 280-4000 cm −1 range. A semiconductor parameter analyser system (Keithley 4200A) was used to measure current-voltage (I-V) characteristics.

Gas sensors fabrication and sensing measurement
Initially, suitable amount of the as-prepared WO 3 nH 2 O was combined with an organic to make thick slurry and then was screen printed onto the alumina substrate (sensor dimensions = 1 × 1 cm) and dried under IR lamp. Later the prepared sensors were sintered at 400°C to eliminate organic residues and stabilize the sensing signals. The sensor's sensing characteristics were evaluated using a table-top static gas-sensing apparatus, model no. TPD-BARC-16CH [37]. While air and target gas went through the sensor chamber, the sensor's steady resistance in air was found to be R air and the sensor's steady resistance in the air-gas combination was determined to be R gas . The response of the NO 2 sensor was determined using the R gas /R air ratio.

Results and discussion
3.1. X-ray diffraction studies X-ray diffraction (XRD) patterns were used to investigate the phase purity and crystallographic structure of nanomaterial prepared at various hydrothermal temperatures. XRD patterns of synthesized WO 3 · nH 2 O materials obtained at various hydrothermal temperatures are shown in figure 1. The products  Further, it was also noted that as the hydrothermal temperature was increased, the intensity of the diffraction peaks and crystallinity of the material were also increased, indicating the improvement in the crystallinity of hydrated WO 3 . This is most likely due to the fact that the increased dissolution rate in hydrothermal system at elevated temperature favours the crystal development of WO 3 · nH 2 O microstructures.

Raman scattering spectroscopy
The degree of tungsten oxide hydration and the molecular vibration of the samples can both be determined using Raman spectroscopy. This information is helpful to explain the structural changes that take place in the material at various synthesis temperatures. The Raman spectra of W1, W2 and W3 nanostructures are shown in figure 5a. The existence of a band at about 946 and 636 cm −1 in the Raman spectra for the all the samples reflects the primary characteristics that are typical of hydrated WO 3 [38]. The peak at 660 cm −1 , which is strongly impacted by hydration, is representative of levels of hydration present in the samples, which is also confirmed from XRD analysis. When the synthesis temperature is between 160°C (W2) and

Morphological analysis
The morphology of hydrated WO 3 powder formed at various temperatures is depicted in figure 2. It can be seen that the hydrothermal synthesis parameters have significant influence on the morphology, particle size, as well as agglomeration of the nanostructures. The WO 3 · H 2 O powder synthesized at 120°C (W1) formed by stacking of small sheet is observed and is around 50-80 nm in length, as seen from figure 2a. When the temperature is elevated from 120°C to 160°C (W2), the WO 3 · H 2 O sheets grew longer (0.7-1.2 µm in length) and arrange in flower-like pattern. Further, WO 3 synthesized at 200°C (W3) exhibited a more consistent flower-like structure than W2, resulting in a greater surface area of the partially hydrated WO 3 powder. This demonstrates that high temperatures provide more energy for hydrothermal processes and promote hydrated WO 3 crystal formation. As a result, particle size also increases figure 3.

Growth mechanism
It is important to study a plausible growth mechanism might be responsible for the development of the uniform WO 3 · nH 2 O nanosheet staked in flower-like pattern shown in figure 4, The reaction steps may be evaluated as shown below: To reveal the growth mechanism of WO 3 · nH 2 O, a series of experiments with varying reaction temperatures were carried out while the other reaction parameters remain constant. The morphology of WO 3 microstructures is greatly influenced by the hydrothermal temperature. Low hydrothermal temperatures result in uneven and aggregated nanostructures of varying sizes; however, increase in the hydrothermal temperature has significant effect and is favourable environment for the formation of nanosheets. This could be related to the effect of K 2 SO 4 and H 2 C 2 O 4 as structurally guiding agents. Oxalic acid can control the rate of growth of different faces of WO 3 by adsorbing on the surfaces of nanoparticles in different ways. It can also act as a selective adsorbent or capping agent. In the case of royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 221135 K 2 SO 4 , the SO 2À 4 ion can only restrict nucleus growth in two directions, resulting in the formation of a one-dimensional crystal. Furthermore, the quantity of K 2 SO 4 in the particles has a considerable influence on their size. When oxalic acid is used as an additive, two directions of WO 3 · nH 2 O growth    [40,41]. In our experiment, firstly Na 2 WO 4 and HCl interacted to produce H 2 WO 4 (equation (3.1)) [42]. Second, nucleation began and the WO 3 · H 2 O crystal nucleus was developed at the initiation of the hydrothermal process. The induction action of Na + and K + then resulted in a large number of WO 3 · H 2 O primary nanoparticles. When the hydrothermal reaction was carried out for 120°C, nanosheet-based aggregates with round shape were observed. Later, the formation of WO 3 · H 2 O takes with the formation of WO 2.90 . At 160°C, only WO 3 · H 2 O was formed with no impurities of WO 2.90 , and sheets staked in flower-like pattern were observed. When the reaction temperature was extended to 200°C, larger sheets anchored in a flowerlike pattern were formed more consistently. During the Ostwald ripening, WO 3 · H 2 O changed into WO 3 · 0.33H 2 O.

UV-visible analysis
UV-Vis spectroscopy is used to examine the optical characteristics of hydrated WO 3 nanostructures, and the results are depicted in figure 5. The threshold values of W2 and W3 were red-shifted in contrast with W1. This red shift in threshold values promotes free carrier formation and might be beneficial for enhancing gas-sensing performance. The band gap energy was calculated from K-M model [43] by plotting a graph of (αhʋ) 2 versus photon energy (hʋ) (Tauc plot). The estimated band gap for W1, W2 and W3 was 2.6, 2.4 and 2.2 eV, respectively. Increases in absorbance and particle size suggest a lowering in the band gap of the samples with reaction temperature [44].

Fourier transform infrared spectroscopy analysis
The chemical bonding of WO 3 · nH 2 O microstructure was analysed through FTIR spectroscopy. The FTIR spectra of all the synthesized materials are depicted in figure 6.
A broad absorption band in the wavenumber range of 420-1000 cm −1 confirms the synthesis of tungsten oxide due to the vibration modes of the W-O or W=O bond. The FTIR spectra clearly show that the typical absorption bands of tungsten oxide are noticeable for the nanostructure obtained at 200°C (W3). Hence, it can be concluded that higher reaction temperatures tend to form the essential W-O chemical bonding due to the proper growth environment.  figure 7. When a semiconductor and metal come into contact, a Schottky barrier is formed, which results in rectifying (i.e. nonlinear I-V) connections, which are typically undesirable [45]. Synthesized WO 3 sensors demonstrate linear I-V characteristics with ohmic behaviour for all the synthesized nanostructure sensors. Furthermore, sensors fabricated using W3 nanostructures indicating more connectivity of the grains.

Gas-sensing properties
To study the gas-sensing measurement of synthesized hydrated WO 3 nanostructures, initially, the operating temperature was optimized, as operating temperature is one of the most essential   royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 221135 parameters that determine the precise kinetics that takes place among both NO 2 and sensitive materials. The sensor response of the fabricated sensors was examined at the optimized operating temperature of 185°C. The behaviour of these gas sensors was plotted and its performance was calculated, as shown in figure 8. Figure 8 shows a remarkable change in resistance when the NO 2 gas was exposed to all the fabricated sensors (W1, W2 and W3) over the time.
The sensor response increases as the hydrothermal temperature rises. The effect of hydrothermal temperature on detecting ability may be linked to its influence on the morphology and structure of the materials. Hydrothermal temperature modifies morphology of the material, influences its form and defines its unique surface area. As a result of this, large number of active sites was available for NO 2 adsorption and enhances gas molecule flow in the sensitive layer. Therefore, W3 sensor  The response and recovery characteristics of gas sensors are also essential parameters. In practical applications, particularly real-time monitoring, sensors must have good response and recovery properties. Response and recovery times are known as the time taken by the sensor to accomplish 90% of entire electrical resistance change in the case of adsorption and desorption, respectively. The response/recovery times of the W1, W2 and W3 sensors towards 25 ppm NO 2 gas are 98 s/143 s, 75 s/107 s and 35 s/83 s, respectively (figure 9). These obtained results indicate that W3 sensor is considerably faster than those of the W1 and W2 sensors.
The huge surface area of the sheets staked in a flower-like pattern supplied enough active sites, resulting in a quick gas-surface interaction. However, once the sensor was exposed to the air environment again, the resistance progressively dropped (83 s) to a near-baseline resistance. It is well recognized that fabricating a sensor that is capable of sensing gas at small amounts and allowing gas measurement across a large content is of practical interest. Hence, W1, W2 and W3 sensors were evaluated in 5-100 ppm NO 2 gas at optimum operating temperature of 185°C to study their sensing properties. The response-recovery curves of W1, W2 and W3 sensors to varied NO 2 gas concentrations (5-100 ppm) are shown in figure 10a. It can be seen that W3 sensor has shown best sensitivity as compared with other sensors. When exposed to 5, 25, 30, 50, 75 and 100 ppm NO 2 gas, the corresponding response values of the sensor are 7.98, 17.89, 18.21, 31.45, 40.55 and 66.22, which are much larger than those of the W1 and W2 sensors.
Selectivity is another important property for gas sensors in practical implementations. Figure 10b depicts the sensor's reactions to several testing gases such as SO 2 , NH 3 , ethanol and NO 2 .
The obtained results show that the response of the W1, W2 and W3 sensor to NO 2 is greater than that of the other gases, demonstrating that the sensor has a good selectivity for NO 2 . The reason behind the smaller response might be explained as, at given temperature, the adsorbed oxygen on the WO 3 surface and gases such as NH 3 and CO could not interact strongly, hence the sensors had a limited response for these gases. Moreover, because NO 2 has a great affinity for electrons, it may acquire electrons directly from the conduction band as well as react with adsorbed oxygen to obtain electrons at a given temperature. As a result of the selective adsorption and electron exchange of NO 2 on the WO 3 · nH 2 O surfaces, considerably good selectivity was achieved. From figure 10c, the sensor royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 221135 response exhibits a linear behaviour as a function of the NO 2 concentration in the log-log plot with the detection limit to 91 ppb.
The effectiveness of WO 3 and WO 3 · H 2 O-based sensors for the detection of various gases is summarized in table 1. According to the findings of a prior research, it was observed that our fabricated sensor in this work operates at a lower temperature than that of the reported literatures. Also, our sensors have a lower detection limit, higher response and quicker response and recovery time than other WO 3 · H 2 O-based sensors. The novelty of this work lies in the fact that to the best of our knowledge WO 3 · H 2 O sensors have not before been used to detect NO 2 .
As a result, this procedure is simple to carry out and easily accessible. With all of these positive attributes, we may conclude that our sensor performance is significantly better than the other mentioned in the literature.

Gas-sensing mechanism
The sensing abilities of resistive-type metal oxide gas sensors are strongly determined by the change in resistance of the sensing material as a result of gas contact, where resistance varies according to analyte type and sensor material characteristics. The contact of gas with the surface of flower-like WO 3 · nH 2 O involved two phenomena, namely adsorption and desorption of atmospheric oxygen molecules on the surface, followed by electrostatic interaction between oxygen species and sensing materials [51,52]. The proposed gas-sensing mechanism is described using schematic diagram as shown in figure 10. When the gas sensor reacts with oxygen, oxygen molecules pick up electrons from the material surface and become charged, resulting in adsorbed oxygen species (   Once enough oxygen molecules have been absorbed, charge depletion layer forms on the material grains. It is possible to produce steady-state surface oxygen content in air by giving a simple electrical resistance. When the sensor is exposed to NO 2 , it interacts with the adsorbed oxygen species on the surface of the sensing film, trapping electrons from the conduction band and lowering the electron density. It results in the widening of the potential barrier, as seen in figure 11. The following equation represents the sensor surface. Due to the strong electron affinity of compared with the oxygen  Figure 11. Gas-sensing mechanism.
royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 10: 221135 molecule, the interaction between the gas molecule and adsorbed oxygen species enhanced, resulting in an increase in sensor resistance. NO 2ðgasÞ þ e À ! NO À 2ðadsÞ ð3:9Þ and NO 2ðgasÞ þ O À þ 2e À ! NO ¼ 2ðadsÞ þ O 2À ðadsÞ : ð3:10Þ The optimized temperature of the WO 3 sensor in the current work is 185°C; therefore, adsorbed oxygen might be present in O − form. When the flow of NO 2 gas is terminated, NO 2(ads) ions are desorbed. As a result, the initial situation is re-established. The cycle reactions continue, and therefore NO 2 detection is accomplished.

Conclusion
In the present work, WO 3 · nH 2 O microstructures were successfully synthesized by using hydrothermal technique at various reaction temperatures (120-200°C). The materials' XRD patterns indicated the monoclinic WO 3 · H 2 O crystal phase transforms into orthorhombic WO 3 · 0.33H 2 O as the synthesis temperature increased and the morphological studies showed nanosheet agglomerated to form flowerlike structures. Gas-sensing measurements showed that nanostructures synthesized at 200°C (W3) showed higher sensor response of 17.89 towards 25 ppm NO 2 at 185°C.
All authors gave final approval for publication and agreed to be held accountable for the work performed therein.
Conflict of interest declaration. We declare we have no competing interest. Funding. This work was supported Science and Engineering Research Board (SERB) curriculum for financing under the core research grant (grant no. CRG/2019/004990).